Substrates used in printed circuit boards are generally comprised of polymeric coatings over a base material. The substrates may be rigid or flexible. Typical base materials used in the industry may be metallic or non-metallic such as steel or fiberglass mesh respectively. In the case of flexible circuits, base materials are generally very thin and the coating applied thereon is also very thin so as to allow for flexibility of the printed circuit substrate. Other substrates are also known in the art.
Typical coatings over the base materials for these printed circuits include phenolic resins, phenolic-rubber resin, ABS and various epoxy resins. It has been found, especially with the use of epoxy resins, that in certain instances, especially with flexible circuit boards having epoxy coatings for the substrate, a problem exists which may be termed solder sticking.
During the manufacture of a printed circuit assembly on a printed circuit substrate, solder is generally caused to be flowed over the substrate surface after the formation of the circuit pattern thereon. This solder can be flowed over the surface during the final stages of printed circuit manufacture, such as in the roll-tinning or hot-gas solder leveling processes or can be applied during assembly of the printed circuit board with its associated electronic components in the wave soldering, solder dipping or drag soldering operations. All of the above soldering techniques are well known in the art. Ordinarily, the solder sticks only to the metallic circuit pattern applied to the substrate. However, occasionally solder is found to stick on the substrate in areas between the circuit pattern. If the solder sticking is extensive or occurs between closely spaced circuit lines, a short or leakage path may occur in the circuit. This problem especially manifests itself in the newer circuit board technology wherein spacing between lines is often kept at a minimum.
There have been various efforts to control solder sticking involving many approaches including modifying the solder or soldering parameters, fluxes and the like. We have discovered that a technique which is generally employed and related to improving adhesion, can unexpectedly be used to prevent adhesion or sticking of solder. The technique referred to is that of roughening the surface onto which the material is to be coated. Generally, the literature teaches that adhesion of a coating on a surface is improved by roughening of the surface. This would normally lead one away from the use of such a technique for the prevention of adhesion or sticking of a material to a surface.